For example, an uninterruptible power supply unit (UPS) is a power supply unit that normally charges a storage battery by a commercial AC power supply and upon electric outage, converts the power stared in the storage battery to an AC power and outputs the AC power. The uninterruptible power supply unit is used for, as well as such backup for electric outage, utilizing a midnight power during daytime, or utilizing, during the night, a power obtained from a stand-alone power outlet (AC) of a power conditioner for photovoltaic generation, for example. Thus, the uninterruptible power supply unit is widely used also for effective utilization of a power.
Such an uninterruptible power supply unit is connected to, for example, a commercial power supply outlet, and an electric apparatus as a load is connected to a power outlet provided to the uninteruptible power supply unit. Normally, power is supplied from a commercial power supply directly to the load via a bypass AC path in the uninterruptible power supply unit, and a storage battery is charged in the uninterruptible power supply unit. Charging of the storage battery is performed via a converter that performs AC-to-DC conversion, and discharging of the storage battery is performed via an inverter that performs DC-to-AC conversion (see, for example, Non Patent Literature 1). The converter on the input side with respect to the storage battery and the inverter on the output side with respect to the storage battery do not operate at the same time, and therefore the input side and the output side are isolated from each other.
On the other hand, a bidirectional inverter can be used in common for charging and discharging of the storage battery. In this case, in charging of the storage battery, the bidirectional inverter operates as a converter for performing AC-to-DC conversion, and in discharging of the storage battery, the bidirectional inverter operates as an inverter for performing DC-to-AC conversion. An AC switch is provided for disconnecting the bidirectional inverter and the commercial power supply from each other in discharging of the storage battery (see, for example, Patent Literature 1).